Method of making a thermionic device



March 29, 1966 R. F. HILL ETAL 3,243,292

METHOD OF MAKING A THERMIONIC DEVICE Original Filed May 14, 1962 INVENTOR 5 1 1 042921 Z 7567/, BY 52022 Jpapmalg one/a .L? rffY/er UnitedStates Patent METHOD OF MAKING A THERMIONIC DEVICE Robert F. Hill,Warren, MiClL, and Stan .l. Paprocki and Donald L. Keller, Columbus,Ohio, assignors, by direct and mesne assignments, to General MotorsCorporation, Detroit, Mich, a corporation of Delaware Originalapplication May 14, 1962, Ser. No. 194,448.

Divided and this application Nov. 5, 1964, Ser. No.

3 Claims. 01. 7s 203 This application is a division of our copendingapplication S.N. 194,448, Robert P. Hill et -al., entitled ThermionicDevice, which was filed May 14, 1962.

This invention relates to devices for converting heat energy toelectrical energy and, more particularly to an improved electron emitterelement for such devices.

In United States patent application Serial No. 802,958, now Patent No.3,093,567, filed March 30, 1959 in the names of Francis E. Jablonski andCharles B. Leffert and assigned to the assignees of the presentinvention, there is described and claimed a thermionic converter whichcomprises a noble gas plasma diode, the electron emitter element ofwhich includes a fissionable material so as to generate the noble gasplasma by fission fragment ionization. Further in accordance with thatinvention, the mass of the fissionable material in the diode or acombination of such diodes can be made sufficiently large to sustainchain reaction fission and thereby generate the heat for conversion toelectrical energy.

The basic requirements for the emitter material for such a device are:It must have the ability to adequately supply electrons at the operatingtemperature, must be chemically and mechanically stable, must haveelectrical and thermal conductivity and must have a high surface densityof exposed fissionable material.

A further object of the invention is to provide an improved method formaking such an electron emitter.

Uranium carbide, by itself, has two of the aforementioned essentialqualities needed for the electron emitter, namely, ability to supplyelectrons and high density of fissionable material. However, by itselfit has insufficient mechanical strength. Uranium carbide is, of course,a ceramic and, as is true of most all ceramics, it is particularlysusceptible to thermal cracking. In accordance with the presentinvention, the additional required properties are provided by physicallycombining certain metals with the uranium carbide to form a cermet. Wehave found that the choice of metals is extremely limited for the reasonthat uranium carbide is quite reactive at high temperatures, and it isessential that the metal not undergo interaction with the uraniumcarbide either during processing or during operation. More specifically,we have found that the two metals which suffice are rhenium andtungsten. Briefly then, the electron emitter of this invention comprisesa dense cermet body of uranium carbide and rhenium, tungsten, orrhenium-tungsten alloys, the former being preferred. Further inaccordance with the invention, this cermet body is provided with aniobium cladding, a thin layer of the rhenium or tungsten, preferablythe latter, being interposed between the cermet body and the cladding inorder to prevent diffusion and reaction between the niobium and theuranium carbide during manufacture and operation of the emitter. Thecermet body can contain from 10% to 80% by volume uranium carbide, theupper end of this range being preferred particularly where the emitteris for use in a nuclear reactor type thermionic converter wherein theheat is supplied by nuclear fission.

The above and other objects and features of the invention will appearmore clearly from the following detailed description thereof made withreference to the Patented Mar. 29, 1966 drawing which shows a side viewin section of an electron emitter constructed in accordance with theinvention and incorporated in a noble gas plasma diode which is shownschematically.

Referring now to the drawing, the thermionic converter comprises ahermetically sealed envelope 2 filled with a noble gas and having twoelectrical leads 4 and 6 extending therethrough. The top lead 4 connectsto an electron collector 8 which has some suitable cooling meansassociated therewith, such cooling means being illustrated by theplurality of heat radiating fins 10. The bottom electrical lead 6 isconnected to the electron emitter 12 which is in the form of a flat discwith its upper surface in spaced relationship to the bottom flat surfaceof the collector. Hence, when the noble gas between the emitter and thecollector is ionized and the emitter is heated, there is a flow ofelectrons from the emitter to the collector thereby generating anelectrical current, all as described in the aforementioned patentapplication.

In accordance with the present invention the emitter 12 comprises adense cermet body 14 of by volume uranium carbide and 20% by volumerhenium with a niobium cladding 16 covering its side and bottomsurfaces, a layer of tungsten foil 18 being pressed between the body 14and the cladding. Further details of the structure of the emitter willbe apparent from the following description of the method for itsmanufacture:

To form the uranium carbide, pure uranium metal in the form of smalldiscs or rods can be employed as a starting material. Prior to melting,the surface of the metal should be carefully cleaned by electropolishingor by a careful wash in dilute nitric acid. Spectroscopic carbon in rodform can be employed as the starting carbon material. The rod is firstoutgassed by heating to a temperature of about 2000 C. in high vacuumand is then ground to powder form in an inert atmosphere. Carefullycontrolled quantities of the uranium metal and carbon powder are placedin an arc furnace in a purified argon atmosphere where they arearc-melted together to form a uranium carbide button, A graphite-tippedelectrode is used in order to minimize contamination. The composition ofthe uranium carbide produced will, of course, depend upon the Weights ofuranium metal and carbon powder in the furnace charge. With 4.8 weightpercent carbon, the uranium carbide formed will be predominantly uraniummonocarbide; when slightly more than 4.8 weight percent carbon is used,a second phase with composition UC will appear at the grain boundarieswithin the UC matrix and there will sometimes also appear a third phase,U C The precise stoichiometric composition of the uranium carbide is notimportant to the present invention and hence the term uranium carbide asused herein is intended to comprehend both the mono and dicarbides asWell as the intermediates such as U2C3.

The uranium carbide is pulverized to fine grain size, on the order ofminus 300 mesh, preferably in a dry box since the pulverized uraniumcarbide is pyrophoric. Next the pulverized uranium carbide is uniformlyadmixed with rhenium powder, also about minus 300 mesh grain size. inthe proportions desired, preferably 80% uranium carhide and 20% rhenium.This mixing of the powders should also be performed in a dry box and forfurther protection against the pyrophoric nature of the uranium carbideit is desirable that about 1% by weight of a suitable organic materialsuch as Carbowax (polyethylene glycol) be included in the mixture. Inaddition to coating the grains of uranium carbide and thereby protectagainst combustion, the Carbowax also serves as a binder for themixture.

The powder mixture is then cold pressed in a steel die at about 60,000pounds per square inch to thereby form a green compact having a densityof approximately 70% theoretical.

The green compact so formed is inserted snugly into a niobium cup linedwith a thin layer of tungsten foil and this assembly is then heated in avacuum sufficiently to drive out the Carbowax. A niobium lid, preferablyalso lined with tungsten foil, is then placed over the niobium cup andis bonded to the cup by electron beam welding to effect an hermeticseal. Since the electron beam welding is performed in a vacuum, theinterior of the cup is in an evacuated state at the conclusion of thissealing operation. The assembly should preferably be, leakchecked afterthe welding to make certain that it is hermetically sealed.

The resulting niobium encapsulated green compact is placed in anautoclave and is pressure bonded in a helium atmosphere at 10,000 poundsper square inch pressure and 2700 F. This heating and isostatic pressingoperation causes the niobium encapsulation to collapse and the greencompact to sinter and form a dense cermet body. At the conclusion of theoperation the cermet body has a density which is about 99% oftheoretical, After removing from the autoclave and then cooling, the topor lid portion of the niobium cladding is sliced away so as to form thestructure as shown in the drawing.

As indicated above, both rhenium and tungsten have extremely lowchemical reactivity with uranium carbide. Between the two, rhenium ispreferable because it has the lowest chemical reactivity; however, it isless desirable because of its higher nuclear cross section. It isbecause of this that rhenium with its extremely low reactivity is thepreferred metal for the cermet, Whereas tungsten with its lower nuclearcross section is preferred for the barrier layer between the cermet andthe cladding. The somewhat higher reactivity of the tungsten serves tono serious disadvantage where it is used as the barrier layer and thelower cross section outweighs what little disadvantage there is.

It will be understood that while the particulars of the invention havebeen described specifically with reference to a preferred embodimentthereof, various modifications may be made, all Within the full andintended scope of the claims which follow.

We claim:

1. A method for making an electron emitter for a thermionic devicecomprising the steps of forming a uniform mixture of from to 80% byvolume pulverant uranium carbide and from 20% to 90% by volume of ametal selected from the group consisting of rhenium, tungsten andrhenium-tungsten alloys, hermetically encapsulating said mixture in aniobium cladding lined with a metal selected from the aforesaid group,heating and pressing the encapsulated mixture to cause said mixture tosinter to a dense cermet body and then cutting away a partion of theniobium cladding to expose the cermet body.

2. A method for making an electron emitter for a thermionic devicecomprising the steps of forming a uniform mixture of from 10% to byvolume pulverant uranium carbide and from 20% to by volume of a metalselected from the group consisting of rhenium, tungsten andrhenium-tungsten alloys, pressing said mixture to form a green compact,hermetically encapsulating said green comp-act in a niobium claddinglined with a metal selected from the aforesaid group, heating andpressing the encapsulated compact to cause said compact to sinter to adense cermet body and then cutting away a portion of the niobiumcladding to expose the cermet body.

3. A method for making an electron emitter for a therrnionic devicecomprising the steps of forming in a dry atmosphere a uniform mixture offrom 10% to 80% by volume puverant uranium carbide and from 20% to 90%by volume of a metal selected from the group consisting of rhenium,tungsten and rhenium-tungsten alloys, a small but effective amount of anorganic material also being included in said mixture to coat thepulverant uranium carbide and serve as a binder, pressing said mixtureto form a green compact, hermetically encapsulating said green compactin a niobium cladding lined with a metal selected from the aforesaidgroup, said encapsulating step being performed in a vacuum so that theencapsulated compact is evacuated of gas at the conclusion of the step,heating and isostatically pressing the encapsulated compact to causesaid cladding to collapse and said compact to sinter to a dense cermetbody and then cutting away a portion of the niobium cladding to exposethe cermet body.

References Cited by the Examiner UNITED STATES PATENTS 2,943,933 7/1960Lenhart 75-214 3,091,581 5/1963 Barr et al. 75203 X 3,147,362 9/1964Ramsey et al. 29--420.5 X 3,168,399 2/1965 Takohashi et al. 75226 X3,184,840 5/1965 Byrne et al 75- v LEON D, ROSDOL, Primary Examiner.

CARL D. QUARFORTH, Examiner.

R. L. GRUDZIECKI, Assistant Examiner.

1. A METHOD FOR MAKING AN ELECTRON EMITTER FOR A THERMIONIC DEVICECOMPRISING THE STEPS OF FORMING A UNIFORM MIXTURE OF FROM 10% TO 80% BYVOLUME PULVERANT URANIUM CARBIDE AND FROM 20% TO 90% BY VOLUME OF AMETAL SELECTED FROM THE GROUP CONSISTING OF RHENIUM, TUNGSTEN ANDRHENIUM-TUNGSTEN ALLOYS, HERMETICALLY ENCAPSULATING SAID MIXTURE IN ANIOBIUM CLADDING LINED WITH A METAL SELECTED FROM THE AFORESAID GROUP,HEATING AND PRESSING THE ENCAPSULATED MIXTURE TO CAUSE SAID MIXTURE TOSINTER TO A DENSE CERMET BODY AND THEN CUTTING AWAY A PARTION OF THENIOBIUM CLADDING TO EXPOSE THE CERMET BODY.